Refactor footnote handling

- Remove unused footnote section title option
- Move away from maintaining a list of footnotes in the document (only needed
  for html export, potential maintainance overhead when modifying the document)
  and rather only build it on export when required.
- HTML export: Rename all footnotes to numbers (so we can support anonymous
  footnote references by assigning them a number) and export footnotes in order
  of reference, not definition. The implementation of this makes it natural to
  also stop exporting unused footnote definitions so we do that as well.

org/inline.go

@@ -171,7 +171,6 @@ func (d *Document) parseFootnoteReference(input string, start int) (int, Node) {
 		link := FootnoteLink{name, nil}
 		if definition != "" {
 			link.Definition = &FootnoteDefinition{name, []Node{Paragraph{d.parseInline(definition)}}, true}
-			d.addFootnote(name, link.Definition)
 		}
 		return len(m[0]), link
 	}